Thin film transistor substrate

ABSTRACT

A thin film transistor includes a gate electrode, a semiconductor layer, and source and drain electrodes contacting the semiconductor layer. The source and drain electrodes include a metal oxide having a crystal size in a c-axis direction Lc(002) that ranges from 67 Å or more to 144 Å or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Division of U.S. patent application Ser. No. 14/691,092, filed on Apr. 20, 2015, which claims priority from and the benefit of Korean Patent Application No. 10-2014-0161644, filed on Nov. 19, 2014, which are hereby incorporated by reference for all purposes as if fully set forth herein.

BACKGROUND Field

Exemplary embodiments of the present invention relate to a thin film transistor substrate.

Discussion of the Background

In general, a flat panel display device, such as a liquid crystal display device or an organic light emitting display device, includes pairs of electric field generating electrodes, and electro-optical active layers interposed therebetween. The liquid crystal display device typically includes a liquid crystal layer as an electro-optical active layer, and the organic light emitting display device typically includes an organic light emitting layer as an electro-optical active layer.

One of the electric field generating electrodes of the pair is generally connected to a switching element to receive an electric signal, and the electro-optical active layer converts the electric signal to an optical signal to display an image.

The flat panel display device generally uses a thin film transistor (TFT) including a three-terminal element as a switching element, signal lines such as gate lines for transmitting a scan signal for controlling the thin film transistor, and data lines for transmitting a signal applied to a pixel electrode.

The above information disclosed in the Background section is only for enhancement of understanding of the background of the inventive concept, and, therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY

Exemplary embodiments of the present invention provide a thin film transistor substrate with an improved yield.

Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention.

An exemplary embodiment of the present invention discloses a thin film transistor, including a gate electrode; a semiconductor layer; and source/drain electrodes that are in contact with the semiconductor layer, the source/drain electrodes including a metal oxide having a crystal size in a c-axis direction Lc(002) in a range between 67 Å and 144 Å.

Another exemplary embodiment of the present invention discloses a thin film transistor, including a gate electrode; a semiconductor layer; and a capping layer including first metal oxide of which a crystal size in a c-axis direction between 67 Å and 144 Å; a barrier layer including a second metal oxide of which a crystal size in the c-axis direction Lc(002) is smaller than 67 Å; and source/drain electrodes including a metal layer disposed between the capping layer and the barrier layer.

Another exemplary embodiment of the present invention discloses a thin film transistor, including a gate electrode; a semiconductor layer; and source/drain electrodes including a metal layer and a barrier layer, the barrier layer being interposed between the metal layer and the semiconductor layer and including metal oxide of which a crystal size in a c-axis direction between 67 Å and 144 Å.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention.

FIG. 1 is a schematic layout diagram of a thin film transistor substrate according to exemplary embodiments of the present invention.

FIG. 2 is a schematic cross-sectional view taken along line of FIG. 1.

FIG. 3 is a relationship graph of tip length and crystal size Lc(002) in a c-axis direction of a capping layer of FIG. 1.

FIGS. 4, 5, 6, 7, and 8 are scanning electronic microscope (SEM) pictures of source/drain electrodes of FIG. 1.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Advantages and features of the present invention and methods of accomplishing the same may be understood more readily by reference to the following detailed description of preferred embodiments and the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art, and the present invention will only be defined by the appended claims. Like reference numerals refer to like elements throughout the specification.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It will be understood that when an element or layer is referred to as being “on”, “connected to” or “coupled to” another element or layer, it can be directly on, connected or coupled to the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on”, “directly connected to” or “directly coupled to” another element or layer, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.

Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

Embodiments are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures). As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, these embodiments should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the present invention.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this specification and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic layout diagram of a thin film transistor substrate according to exemplary embodiments of the present invention. FIG. 2 is a schematic cross-sectional view taken along line II-IF of FIG. 1.

Referring to FIGS. 1 and 2, a thin film transistor substrate according to exemplary embodiments of the present invention may include an insulation substrate 110, gate lines GL, gate electrodes 124, data lines DL, source electrodes 173, drain electrodes 175, a semiconductor layer 151, pixel electrodes 191, and the like.

The thin film transistor substrate may have a structure in which the gate line GL, the gate electrode 124, the semiconductor layer 151, the source electrode 173, the drain electrode 175, and the pixel electrode 191 are sequentially stacked on the insulation substrate 110.

The insulation substrate 110 may be formed of transparent glass or a composite resin.

The gate line GL may transmit a gate signal, and be extended on the insulation substrate 110 in a substantially horizontal direction. The gate electrode 124 may protrude from the gate line GL toward the pixel electrode 191. For example, the gate line GL and the gate electrode 124 may be formed of an aluminum based metal, such as aluminum (Al) and an aluminum alloy, a silver based metal, such as silver (Ag) and a silver alloy, a copper based metal, such as copper (Cu) and a copper alloy, a molybdenum based metal, such as molybdenum (Mo) and a molybdenum alloy, chrome (Cr), titanium (Ti), tantalum (Ta), etc.

The data line DL may transmit a data signal, and may extend on the insulation substrate 110 in a vertical direction. The source electrode 173 may protrude from the data line DL, and partially overlap the gate electrode 124. The source electrode 173 may have a “U” shape as illustrated in the drawings, but the shape is not limited thereto. The drain electrode 175 is disposed separate from the source electrode 173. The drain electrode 175 may partially overlap the gate electrode 124. For example, the drain electrode 175 may be disposed within a valley of the “U”-shaped source electrode 173. The drain electrode 175 may be in contact with the pixel electrode 191 through a contact hole 185.

Each of the data line DL, the source electrode 173, and the drain electrode 175 may have a stacked structure including capping layers 173 r and 175 r, metal layers 173 q and 175 q, and barrier layers 173 p and 175 p. The metal layers 173 q and 175 q may be disposed between the capping layers 173 r and 175 r and the barrier layers 173 p and 175 p.

The metal layers 173 q and 175 q may serve as a main wire layer for transmitting a data signal. For example, the metal layers 173 q and 175 q may be formed of nickel (Ni), cobalt (Co), titanium (Ti), silver (Ag), copper (Cu), molybdenum (Mo), aluminum (Al), beryllium (Be), niobium (Nb), gold (Au), iron (Fe), and/or the like.

The capping layers 173 r and 175 r may prevent the metal layers 173 q and 175 q from being oxidized, and the barrier layers 173 p and 175 p may prevent metal elements forming the metal layers 173 q and 175 q from being diffused. The barrier layers 173 p and 175 p may serve as ohmic contact layers.

The capping layers 173 r and 175 r and the barrier layers 173 p and 175 p will be described in detail with reference to FIGS. 3 to 8 below.

The pixel electrode 191 may be disposed within a region where the gate line GL and the data line DL cross each other. For example, the pixel electrode 191 may be a transparent electrode formed of an indium-titanium oxide (ITO) or an indium-zinc oxide (IZO). The pixel electrode 191 may receive a data voltage from the drain electrode 175 through the contact hole 185.

The semiconductor layer 151 may be disposed on the gate electrode 124, and under the source electrode 173 and the drain electrode 175. The semiconductor layer 151 may be an oxide semiconductor layer. The semiconductor layer 151 may include at least one of zinc (Zn), indium (In), tin (Sn), gallium (Ga), and hafnium (Hf). For example, the semiconductor layer 151 may be formed of an indium-gallium-zinc oxide (IGZO). The semiconductor layer 151 may include a protrusion 154. The protrusion 154 may be partially exposed between the source electrode 173 and the drain electrode 175.

Referring to FIG. 2, a gate insulation layer 140 may be disposed between the insulation substrate 110 and the gate electrode 124. The gate insulation layer 140 may cover the entire surface of the gate electrode 124. The gate insulation layer 140 may include a first insulation layer 140 a and a second insulation layer 140 b. For example, the first insulation layer 140 a may be formed of silicon nitride (SiNx) having a thickness of about 4,000 Å, and the second insulation layer may be formed of silicon oxide (SiO₂) having a thickness of about 500 Å. The first insulation layer 140 a may be formed of silicon oxynitride (SiON) and the second insulation layer 140 b may be formed of silicon oxide (SiO₂), but is not limited thereto. The gate insulation layer 140 may also be configured as a single layer unlike the illustration, but this is merely exemplary and is not limited thereto.

The semiconductor layer 151 may be disposed on the gate insulation layer 140. The semiconductor layer 151 may be formed in a region overlapping the gate electrode 124. The semiconductor layer 151 may be partially covered by the source electrode 173 and the drain electrode 175. The protrusion 154 may be in contact with a passivation layer. The source electrode 173 and the drain electrode 175 may be disposed on the same plane and may be spaced apart from each other. A passivation layer 180 may be formed on the source electrode 173 and the drain electrode 175.

The passivation layer 180 may be formed of an inorganic insulating material, such as silicon nitride or silicon oxide, an organic insulating material, and/or a low dielectric constant insulating material. The passivation layer 180 may include a first passivation layer 180 a and a second passivation layer 180 b. For example, the first passivation layer 180 a may be formed of silicon oxide, and the second passivation layer 180 b may be formed of silicon nitride. The pixel electrode 191 may be disposed on the passivation layer 180.

Hereinafter, the capping layers 173 r and 175 r and the barrier layers 173 p and 175 p will be described in detail.

The capping layers 173 r and 175 r may include metal oxide having a crystal size Lc(002) in the c-axis direction that ranges from 67 Å or more to 144 Å or less. The capping layer 173 r and 175 r may prevent the metal layer 173 q and 175 q from being oxidized and corroded.

For example, in a comparative embodiment, when the passivation layer formed of silicon oxide is deposited, the data line DL and the drain electrode 175 may be lifted by copper oxide (CuOx) generated by direct contact between the metal layers 173 q and 175 q formed of copper and the passivation layer 180. Additionally or alternatively, the data line DL and the drain electrode 175 may be corroded when the contact hole 185 is formed in the passivation layer 180. However, according to exemplary embodiments of the present invention, the capping layers 173 r and 175 r are positioned under the passivation layer 180, so that it is possible to prevent the data line DL and the drain electrode 175 from being lifted and corroded.

The capping layers 173 r and 175 r may include metal oxide having a crystal size Lc(002) in the c-axis direction that ranges between 67 Å and 144 Å, and a dopant. When the capping layers 173 r and 175 r include the dopant, a content of metal oxide may be in a range between 70 weight % and 85 weight % based on the total weight of the capping layer 173 r and 175 r, and a content of the dopant may be in a range between 15 weight % and 30 weight % based on the total weight of the capping layers 173 r and 175 r.

When the crystal size Lc(002) in the c-axis direction in a range between 67 Å and 144 Å, and the content of the metal oxide is in a range between 70 weight % and 85 weight % based on the total weight of the capping layers 173 r and 175 r, it is possible to restrict a tip from being generated in the capping layers 173 r and 175 r, and prevent the capping layers 173 r and 175 r from being eroded.

The tip of the capping layers 173 r and 175 r is a sharp tip protruding from surfaces of the capping layers 173 r and 175 r. The tip of the capping layers 173 r and 175 r may be generated during a patterning process of the source/drain electrodes 173 and 175. The tip of the capping layers 173 r and 175 r may be separated from the capping layers 173 r and 175 r by physical pressure, and the separated tip may flow into the thin film transistor substrate to cause a defect when a channel part of the semiconductor layer 151 is formed or stripped during the manufacturing process. Further, the tip of the capping layers 173 r and 175 r may generate a crack in the passivation layer 180.

According to an exemplary embodiment, the metal oxide having the crystal size Lc(002) in the c-axis direction that ranges between 67 Å and 144 Å may be zinc oxide (ZnO), and the dopant may be gallium (Ga), indium (In), aluminum (Al), and the like. The capping layers 173 r and 175 r may be formed of one of a gallium-zinc oxide (GZO) layer, an aluminum-zinc oxide (AZO) layer, and an indium-zinc oxide (IZO) layer.

The gallium-zinc oxide (GZO) layer may include zinc oxide (ZnO) in a range between 70 weight % and 85 weight % and gallium (Ga) in a range between 15 weight % and 30 weight %. The aluminum-zinc oxide (AZO) layer may include zinc oxide (ZnO) in a range between 70 weight % and 85 weight % and aluminum (Al) in a range between 15 weight % and 30 weight %. The indium-zinc oxide (IZO) layer may include zinc oxide (ZnO) in a range between 70 weight % and 85 weight % and indium (In) in a range between 15 weight % and 30 weight %.

The metal oxide having the crystal size Lc(002) in the c-axis direction that ranges from between 67 Å and 144 Å may be fabricated by adjusting deposition power, increasing a flow of inert gas, for example, argon (Ar), or increasing a deposition temperature during a process of depositing the capping layers 173 r and 175 r on the metal layers 173 q and 175 q. The metal oxide having the crystal size Lc(002) in the c-axis direction that ranges from between 67 Å and 144 Å may also be fabricated by a method of depositing the capping layers 173 r and 175 r on the metal layers 173 q and 175 q and then performing a heat treatment.

Table 1 below shows that a crystal size in (002) plane of the zinc oxide (ZnO) may be increased by adjusting deposition power.

TABLE 1 Crystal size Deposition 2 in (002) plane power (kW) FWHM θ(degree) of ZnO (Å) Center 25 0.8884 33.790 103.9 38 0.8544 33.700 108.0 50 1.0484 33.668 88.03 Edge 25 0.8855 33.668 104.2 38 0.9821 33.73 93.99 50 1.0132 33.674 91.09

Referring to Table 1, at the center, a crystal size is increased when deposition power is 38 kW, and a crystal size is decreased when deposition power is 50 kW. By contrast, at the edge, when deposition power is increased from 25 kW, 38 kW, to 50 kW, a crystal size is decreased.

A tip is mainly generated in the capping layers 173 r and 175 r at the edge, so that it is possible to increase a crystal size of the zinc oxide (ZnO) by decreasing deposition power at the edge, compared to the center.

Table 2 below shows that a crystal size in (002) plane of the zinc oxide (ZnO) may be increased by adjusting a flow of Argon (Ar).

TABLE 2 Crystal size Deposition 2 in (002) plane power (kW) Ar(sccm) Ar(Pa) FWHM θ(degree) of ZnO (Å) 50 100 0.33 0.8747 32.791 105.27 50 300 0.98 0.6397 32.824 143.96

Referring to FIG. 2, it can be seen that a crystal size in (002) plane of the zinc oxide (ZnO) is increased in proportion to an increase in a flow of Argon (Ar) under the same deposition power.

FIG. 3 is a graph relating a length of the tip generated in the capping layers 173 r and 175 r to a crystal size Lc(002) in the c-axis direction of the capping layers 173 r and 175 r. Referring to FIG. 3, a horizontal axis represents a crystal size Lc(002) in the c-axis direction and a vertical axis represents a length of the tip of the capping layer 173 r and 175 r. As the thin film transistor substrates used in the exemplary embodiment shown in FIG. 3, indium-gallium-zinc oxide (IGZO) is used in the semiconductor layer, copper is used in the metal layer of the source/drain electrodes, and indium-zinc oxide (IZO), in which a weight ratio of zinc oxide (ZnO) and indium oxide (In₂O₃) is 8:2, is used in the barrier layer and the capping layer. Unlike the barrier layer, a crystal size in (002) plane of the zinc oxide of the capping layer is increased by adjusting a deposition temperature, deposition power, and the like, as described above.

Referring to FIGS. 3 to 8 and Table 3 below, when the crystal size Lc(002) in the c-axis direction is 64.5 Å, a length of the tip is 900 Å (see FIG. 4). By contrast, when the crystal size Lc(002) in the c-axis direction is 67.5 Å, a length of the tip is 861 Å (see FIG. 5), when the crystal size Lc(002) in the c-axis direction is 76.6 Å (see FIG. 6), a length of the tip is 603 Å, when the crystal size Lc(002) in the c-axis direction is 76.9 Å, a length of the tip is 667 Å (see FIG. 7), and when the crystal size Lc(002) in the c-axis direction is 108.0 Å, a length of the tip is 0 Å (see FIG. 8). It can be seen from the result of the experiment that when the crystal size Lc(002) in the c-axis direction is increased, a length of the tip is decreased.

TABLE 3 Existence of tip protruding Crystal size Lc(002) in Length (Å) over a boundary surface of c-axis direction (Å) of tip GZO layer and Cu layer 64.5 900 Existence (439 Å) 67.5 861 Non-existence 76.6 603 Non-existence 76.9 667 Non-existence 108.0 0 Non-existence

When the crystal size Lc(002) in the c-axis direction is 64.5 Å, a length of the tip protruding over a boundary surface of the GZO layer and the Cu layer is 439 Å (see FIG. 4). By contrast, when the crystal sizes Lc(002) in the c-axis direction are 67.5 Å, 76.6 Å, 76.9 Å, and 108.0 Å, a tip protruding over a boundary surface of the GZO layer and the Cu layer is not observed (see FIGS. 5, 6, 7, and 8).

The crystal size in (002) plane of the zinc oxide (ZnO) in the c-axis direction may be measured by using the Scherrer Equation (Equation (1) below).

$\begin{matrix} {T = \frac{K\;\lambda}{\beta\;\cos\;\theta}} & {{Equation}\mspace{14mu}(1)} \end{matrix}$

In Equation (1), T is a mean size of aligned crystal domains, K is a dimensionless shape factor, λ is an x-ray wavelength, β is a line broadening at half maximum (FWHM) in radians, and θ is a Bragg angle.

The barrier layers 173 p and 175 p may include metal oxide having a crystal size Lc(002) in the c-axis direction that ranges from 67 Å to 144 Å. The metal oxide having the crystal size Lc(002) in the c-axis direction that ranges from 67 Å to 144 Å may be fabricated by increasing deposition power, increasing a flow of inert gas, for example, argon (Ar), or increasing a deposition temperature during a process of depositing the barrier layers 173 p and 175 p on the semiconductor layer 151. The metal oxide having the crystal size Lc(002) in the c-axis direction that ranges from 67 Å to 144 Å may also be fabricated by depositing the barrier layers 173 p and 175 p on the semiconductor layer 151 and then performing a heat treatment.

The barrier layers 173 p and 175 p may include metal oxide having a crystal size Lc(002) in the c-axis direction that ranges from 67 Å to 144 Å, and a dopant.

For example, the barrier layer may be one of bare zinc oxide (ZnO), a gallium-zinc oxide (GZO) layer, an aluminum-zinc oxide (AZO) layer, and an indium-zinc oxide (IZO) layer.

The gallium-zinc oxide (GZO) layer may include zinc oxide (ZnO) in a range between 77.2 weight % and 94.4 weight % and gallium (Ga) in a range of 5.6 weight % to 22.8 weight %. The gallium (Ga) or a gallium oxide (Ga₂O₃) within the content range may prevent an undercut to enable the barrier layer to serve as a diffusion prevention layer, prevent tailing generable in the barrier layer during an etching process, and prevent a short at a boundary of the channel parts of the semiconductor layer 151.

The aluminum-zinc oxide (AZO) layer may include zinc oxide (ZnO) in a range of 50 mol % to 97.5 mol % and aluminum (Al) in a range of 2.5 mol % to 50 mol %. The aluminum (Al) or aluminum oxide (Al₂O₃) within the content range may prevent an undercut to enable the barrier layer to serve as a diffusion prevention layer, prevent tailing generable in the barrier layer during an etching process, and prevent a short at a boundary of the channel parts of the semiconductor layer 151.

The indium-zinc oxide (IZO) layer may include zinc oxide (ZnO) in a range of 10 weight % to 97.5 weight % and indium (In) in a range of 2.5 weight % to 90 weight %. The indium-zinc oxide (IZO) layer may include zinc oxide (ZnO) in a range of 60 weight % to 80 weight % and indium (In) in a range of 20 weight % to 40 weight %. The indium (In) or an indium oxide (In₂O₃) within the content range may prevent an undercut to enable the barrier layer to serve as a diffusion prevention layer, prevent tailing generable in the barrier layer during an etching process, and prevent a short at a boundary of the channel parts of the semiconductor layer 151.

The barrier layers 173 p and 175 p may include a metal oxide having a crystal size Lc(002) in the c-axis direction of smaller than 67 Å. As mentioned above, the metal oxide having a crystal size Lc(002) in the c-axis direction that ranges from between 67 Å to 144 Å may be fabricated by increasing deposition power, increasing a flow of inert gas, for example, argon (Ar), or increasing a deposition temperature during a process of depositing the barrier layers 173 p and 175 p on the semiconductor layer 151. By contrast, the metal oxide having the crystal size Lc(002) in the c-axis direction of smaller than 67 Å may be fabricated by a method, such as a method of decreasing deposition power, a method of decreasing a flow of inert gas, or a method of adjusting a deposition temperature to a room temperature condition.

According to exemplary embodiments of the present invention, in the thin film transistor substrate, it is possible to minimize inflow of impurities generated from source/drain electrodes.

According to exemplary embodiments of the present invention, in the thin film transistor substrate, it is possible to restrict a crack from being generated within a passivation layer.

According to exemplary embodiments of the present invention, in the thin film transistor substrate, it is possible to minimize an undercut of a barrier layer.

The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few embodiments of the present invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of the present invention. Accordingly, all such modifications are intended to be included within the scope of the present invention as defined in the claims. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The present invention is defined by the following claims, with equivalents of the claims to be included therein. 

What is claimed is:
 1. A thin film transistor substrate, comprising: a semiconductor layer; and a barrier layer disposed on the semiconductor layer and comprising a second metal oxide having a crystal size in the c-axis direction Lc(002) less than 67 Å; a capping layer disposed on the barrier layer and comprising a first metal oxide having a crystal size in a c-axis direction LC(002) greater than 67.5 Å and less than 108 Å; source and drain electrodes disposed between the capping layer and the barrier layer, and each of the source and drain electrodes comprising a metal layer.
 2. The thin film transistor substrate of claim 1, wherein each of the first metal oxide and the second metal oxide comprises one of a bare zinc oxide (ZnO) layer, a gallium-zinc oxide (GZO) layer, an aluminum-zinc oxide (AZO) layer, and an indium-zinc oxide (IZO) layer.
 3. The thin film transistor substrate of claim 2, wherein the first metal oxide comprising the gallium-zinc oxide (GZO) layer comprises zinc oxide (ZnO) between 70 weight % and 85 weight % and gallium (Ga) between 15 weight % and 30 weight %, wherein the first metal oxide comprising the aluminum-zinc oxide (AZO) layer comprises zinc oxide (ZnO) between 70 weight % and 85 weight % and aluminum (Al) between 15 weight % and 30 weight %, and wherein the first metal oxide comprising the indium-zinc oxide (IZO) layer comprises zinc oxide (ZnO) between 70 weight % and 85 weight % and indium (In) between 15 weight % and 30 weight %.
 4. The thin film transistor substrate of claim 2, wherein the second metal oxide comprising the gallium-zinc oxide (GZO) layer comprises zinc oxide (ZnO) between 77.2 weight % and 94.4 weight % and gallium (Ga) between 5.6 weight % and 22.8 weight %, wherein the second metal oxide comprising the aluminum-zinc oxide (AZO) layer comprises zinc oxide (ZnO) between 50 mol % and 97.5 mol % and aluminum (Al) between 2.5 mol % and 50 mol %, and wherein the second metal oxide comprising the indium-zinc oxide (IZO) layer comprises zinc oxide (ZnO) between 10 weight % and 97.5 weight % and indium (In) between 2.5 weight % and 90 weight %.
 5. The thin film transistor substrate of claim 4 wherein the second metal oxide comprising the indium-zinc oxide (IZO) layer comprises zinc oxide (ZnO) between 60 weight % and 80 weight % and indium (In) between 20 weight % and 40 weight %. 